Composites comprising reinforcing fibers such as carbon fibers or aromatic polyamide fibers and a matrix resin have high specific strength and rigidity, and are used as structural materials for aircraft. Conventional matrix resins having high heat resistance do not provide composites having high elongation even if the resins are reinforced with fibers having high elongation. A composite having high elongation has been produced by using a properly selected matrix resin composition having a high elongation. However, such matrix resin has poor heat resistance. Therefore, no prior art matrix resin satisfies the two conflicting requirements of high elongation and high heat resistance. In Table 1, the elongation and heat resistance of a composite produced by unidirectionally orienting carbon fibers (elongation&gt;1.5%) impregnated with a conventional heat-resistant matrix resin are compared with those of a composite produced by unidirectionally orienting carbon fibers (elongation&gt;1.5%) impregnated with a conventional high-elongation matrix resin.
TABLE 1 ______________________________________ Matrix Elongation Heat resistance of CFRP* resin at break ILSS** at room temp. ILSS at 100.degree. C. ______________________________________ Heat- 1.2% 13.0 kg/mm.sup.2 10.0 kg/mm.sup.2 resistance Elongative 1.6% 9.8 kg/mm.sup.2 4.7 kg/mm.sup.2 ______________________________________ *Carbon fiber reinforced plastic **Inter laminer shear strength
The strand of carbon fibers used had a strength of 440 kg/mm.sup.2, a modulus of elasticity of 24,100 kg/mm.sup.2 and an elongation of 1.83%. As Table 1 shows, it has been very difficult to produce a composite that is high in both heat resistance and elongation, and this is one of the reasons why composites are conventionally used only as secondary structural members in aircraft. If composite could also be used as primary structural members, the weight of an aircraft could be greatly reduced and significant energy saving would be realized.